Conventionally, there is a need for railroad vehicles such as the Shinkansen bullet train to travel at a higher speed and to have the largest possible transportation capacity. It is thus necessary to reduce the size and the weight of the vehicle body and auxiliary devices, whereas a vehicle-mounted transformer, which has a particularly high mass among the auxiliary devices, is increasing in capacity.
In recent years, there are increasing demands for a low-floor vehicle for the purpose of achieving a barrier-free design. Thus, with regard to underfloor devices such as a vehicle-mounted transformer disposed under the floor of a vehicle such as an AC electric train, there is not only a need for a reduction in size and weight, but also a strong need for a reduction in height to achieve a low-floor vehicle.
WO 2010/092676 (PTD 1) is a prior art document which discloses the configuration of a transformer having a reduced height of the transformer, and in which a decrease in reactance can be prevented. The transformer described in PTD 1 includes a first iron core having a plurality of legs aligned at a distance from each other, a plurality of high-voltage side coils each wound around one of the plurality of legs, and receiving common single-phase AC power, and a plurality of low-voltage side coils provided in correspondence with the high-voltage side coils, magnetically coupled to the corresponding high-voltage side coils, and each wound around one of the plurality of legs. In this transformer, the high-voltage side coils and the low-voltage side coils form a plurality of coil groups. The transformer further includes a second iron core provided between adjacent coil groups.